Editing Well logging (section)

===History===
[[Conrad and Marcel Schlumberger]], who founded [[Schlumberger|Schlumberger Limited]] in 1926, are considered the inventors of electric well logging.  Conrad developed the [[Schlumberger array]], which was a technique for prospecting for [[metal]] [[ore]] deposits, and the brothers adapted that surface technique to subsurface applications.  On September 5, 1927, a crew working for Schlumberger lowered an electric sonde or tool down a well in Pechelbronn, Alsace, France creating the first [[well log]].  In modern terms, the first log was a [[resistivity log]] that could be described as 3.5-meter upside-down lateral log.<ref>{{cite book
  | last = Hilchie
  | first = Douglas W.
  | title = Wireline: A history of the well logging and perforating business in the oil fields
  | publisher = Privately Published
  | year = 1990
  | location = Boulder, Colorado
  | isbn = <!--None
  -->| page = 200 }}</ref>

In 1931, [[Henri George Doll]] and G. Dechatre, working for Schlumberger, discovered that the [[galvanometer]] wiggled even when no current was being passed through the logging cables down in  the well.  This led to the discovery of the [[spontaneous potential]] (SP) which was as important as the ability to measure [[resistivity]].  The SP effect was produced naturally by the borehole [[mud]] at the boundaries of [[Permeability (earth sciences)|permeable]] beds.  By simultaneously recording SP and resistivity, loggers could distinguish between permeable oil-bearing beds and impermeable nonproducing beds.<ref>{{cite journal|last=Pike |first=Bill |author2=Rhonda Duey |title=Logging history rich with innovation |journal=Hart's E&P |pages=52–55 |year=2002 |url=http://www.accessmylibrary.com/coms2/summary_0286-7442038_ITM |access-date=2008-06-02 }}{{dead link|date=June 2016|bot=medic}}{{cbignore|bot=medic}}</ref>

In 1940, Schlumberger invented the [[spontaneous potential]] [[dipmeter]]; this instrument allowed the calculation of the [[strike and dip|dip]] and direction of the dip of a layer.  The basic dipmeter was later enhanced by the resistivity dipmeter (1947) and the continuous resistivity dipmeter (1952).

Oil-based mud (OBM) was first used in Rangely Field, Colorado, in 1948.  Normal electric logs require a conductive or water-based mud, but OBMs are nonconductive.  The solution to this problem was the induction log, developed in the late 1940s.

The introduction of the [[transistor]] and [[integrated circuits]] in the 1960s made electric logs vastly more reliable.  Computerization allowed much faster log processing, and dramatically expanded log data-gathering capacity.  The 1970s brought more logs and computers.  These included combo type logs where [[resistivity logs]] and porosity logs were recorded in one pass in the borehole.

The two types of porosity logs (acoustic logs and nuclear logs) date originally from the 1940s.  [[Sonic logs]] grew out of technology developed during World War II.  Nuclear logging has supplemented acoustic logging, but acoustic or sonic logs are still run on some combination logging tools.

Nuclear logging was initially developed to measure the natural gamma radiation emitted by underground formations.   However, the industry quickly moved to logs that actively bombard rocks with [[nuclear particles]].  The [[gamma ray]] log, measuring the natural radioactivity, was introduced by Well Surveys Inc. in 1939, and the WSI [[neutron]] log came in 1941. The gamma ray log is particularly useful as shale beds which often provide a relatively low permeability cap over hydrocarbon reservoirs usually display a higher level of gamma radiation. These logs were important because they can be used in cased wells (wells with production casing).  WSI quickly became part of Lane-Wells.  During [[World War II]], the US Government gave a near wartime monopoly on open-hole logging to [[Schlumberger]], and a monopoly on cased-hole logging to [[Lane-Wells]].<ref>Now a division of Baker Hughes</ref>  Nuclear logs continued to evolve after the war.

After the discovery of nuclear magnetic resonance by Bloch and Purcell in 1946, the [[nuclear magnetic resonance]] log using the Earth's field was developed in the early 1950s by Chevron and Schlumberger.<ref>{{cite journal |last1=Kleinberg |first1=Robert L. |title=NMR well logging at Schlumberger |journal=Concepts in Magnetic Resonance |date=2001 |volume=13 |issue=6 |pages=396–403 |doi=10.1002/cmr.1026 |url=https://www.researchgate.net/publication/229709476 |access-date=23 September 2020}}</ref>  [[Nicolaas Bloembergen]] filed the Schlumberger patent in 1966.<ref>{{cite journal |last1=Bloembergen |first1=N. |title=Paramagnetic resonance precision method and apparatus for well logging |journal=U.S. Patent 3,242,422 |date=1966}}</ref>  The NMR log was a scientific success but an engineering failure. More recent engineering developments by NUMAR (a subsidiary of [[Halliburton]]) in the 1990s has resulted in continuous NMR logging technology which is now applied in the oil and gas, water and metal exploration industry.<ref>{{cite journal|last1=Kleinberg|first1=Robert L.|last2=Jackson|first2=Jasper A.|title=An introduction to the history of NMR well logging|journal=Concepts in Magnetic Resonance|date=2001|volume=13|issue=6|pages=340–342|doi=10.1002/cmr.1018|doi-access=free}}</ref>{{Citation needed|date=February 2018}}

Many modern oil and gas wells are drilled directionally.  At first, loggers had to run their tools somehow attached to the drill pipe if the well was not vertical.  Modern techniques now permit continuous information at the surface.  This is known as [[logging while drilling]] (LWD) or [[measurement-while-drilling]] (MWD).  MWD logs use [[mud pulse technology]] to transmit data from the tools on the bottom of the [[Drill string|drillstring]] to the processors at the surface.